Radio communication systems are well known in the art. Pursuant to many such systems, an information signal--which may describe a plurality of discrete data elements--is modulated onto a carrier signal and transmitted from a first location to a second location. At the second location, the information signal is demodulated and recovered. Of course, one of the goals of such communication systems is to place a high number of voice channels within a given radio frequency (RF) channel bandwidth while maintaining acceptable recovery levels.
In a digital radio communication system, an input speech wave form is sampled, compressed, encoded, and transmitted through a digital radio communication channel. Prior to transmission, the compressed data elements might be grouped together in a predetermined fashion to form a data transmission unit (TU). For example, digital information can be compressed using vector sum excited linear prediction (VSELP), a well known technique in the digital communication area. FIG. 1 shows a portion of a digital data stream (100), including a VSELP TU (102). The VSELP TU (102) typically carries digital data that is divided into three groups (104-106). The data in the VSELP TU (102) might be divided into groups based on their relative importance to the resultant speech quality. That is, group I data bits might contribute significantly to the overall quality of the decompressed signal, while group 3 data bits may have very little effect on the resultant speech quality. In addition to the data bits shown in data stream (100), control information is also transmitted to the receiver.
Once the data is divided up into groups, the bits may be encoded (e.g., using well known trellis techniques) before being modulated. In particular, data partitioning allows for trellis coding of the data groups at different coding rates. That is, depending on the sensitivity to bit errors (i.e., individual contribution to overall speech quality), an optimal trellis coding rate may be applied to each of the different groups within the same VSELP TU.
Generally, the control information that is transmitted along with the voice information resides in a particular portion of the data stream. However, in some digital radio communication system embodiments, additional control information occasionally replaces a portion of the data stream just prior to being modulated. That is, at some point during the encoding process, the transmitter might need to send additional control information to the receiver (e.g., control information concerning the status or maintenance of a call). Accordingly, this control information is arbitrarily added to the data stream.
To illustrate, FIG. 2. shows an encoded TU (200) after control information (201) is added on top of the normal encoded data packets (e.g., as shown in FIG. 1). This process, commonly referred to as bit stealing, completely compromises the integrity of the data which previously occupied the position that is now occupied by the control information (201) in the encoded TU. That is, because the data information has been overwritten, there is no way to recover that information at the receiving end.
Accordingly, there exists a need for a digital radio communication system that reduces the negative impact that normally results when control information overwrites data information. A system that would allow the voice information to be compromised while reducing the associated degradation in received signal quality and preserving spectral efficiency, would be an improvement over the prior art.